1. Field of the Invention
This invention relates to a plasma display panel, and more particularly to a method and apparatus for driving a plasma display panel that is adaptive for improving a capability of gray scale expression as well as a picture quality.
2. Description of the Related Art
Generally, a plasma display panel (PDP) is a display device utilizing a visible light emitted from a phosphorus material when an ultraviolet ray generated by a gas discharge excites the phosphorus material. The PDP has an advantage in that it has a thinner thickness and a lighter weight in comparison to the existent cathode ray tube (CRT) and is capable of realizing a nigh resolution and a large-scale screen.
Such a PDP drives one frame, which is divided into various sub-fields having a different discharge frequency, so as to express gray levels of a picture. Each sub-field is again divided into a reset period for uniformly causing a discharge, an address period for selecting the discharge cell and a sustain period for realizing the gray levels depending on the discharge frequency. For instance, when it is intended to display a picture of 256 gray levels, a frame interval equal to 1/60 second (i.e. 16.67 msec) is divided into 8 sub-fields. Each of the 8 sub-fields is divided into an address period and a sustain period. Herein, the reset period and the address period of each sub-field are equal every sub-field, whereas the sustain period are increased at a ratio of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field. Since each sub-field has a different sustain period, it is able to express a gray scale of a picture.
However, since the PDP has brightness determined in accordance with the number of sustain pulses, it has problems of picture quality deterioration, power waste and panel damage, etc. when total sustain pulse number in the case of having a high average brightness is equal to that in the case of having a low average brightness If total sustain pulse number is set lowly with respect to all input images, then a contrast is reduced. On the other hand, if total sustain pulse number is set highly with respect to all input images, then the PDP has advantages in that a brightness is high and a contrast is high even in the case of a dark image, but has problems in that a power waste may be increased and a panel may be damaged due to an increase in the temperature of panel. Accordingly, it is necessary to adjust total sustain pulse number appropriately in accordance with an average brightness of an input image.
When the number of sustain pulses corresponding to one gray level interval has a lowest value, that is, ‘1’, the PDP has a higher gray scale expression capability because a gray level expression for an image felt by a human eye becomes closest to a real gray level of an input image, and has an excellent picture quality because an error diffusion artifact is almost not observed by a human eye.
There has been developed a driving scheme of adjusting total sustain pulse number in accordance with an average brightness of an input image, hereinafter referred to as “sustain pulse number control scheme”. Such a conventional sustain pulse number control scheme decreases total sustain pulse number at a place having a high average brightness of an input image, to thereby reduce power consumption and prevent panel damages. On the other hand, the conventional sustain pulse control scheme increases total sustain pulse number when an average brightness of an input image is low, and enhances a contrast at a dark screen.
However, in the conventional sustain pulse control scheme, total sustain pulse number may be adjusted without an increase in total gray level number in accordance with an average brightness of an input image. Thus, there is raised following problems. When the number of sustain pulses only is adjusted without an increase in total gray level number, the number of sustain pulses corresponding to one gray level interval becomes considerably large at a dark image. For instance, if total sustain pulse number is adjusted to ‘768’ when total gray level number is ‘256’ in the conventional sustain pulse control scheme, then the number of sustain pulses corresponding to one gray level interval becomes ‘3’. Since three sustain pulses cause a sustain discharge three times in one gray level interval, a real gray level interval felt by an observer's eye at a dark screen becomes considerably large.
Meanwhile, the PDP has been used a multitoning technique such as an error diffusion or an ordered dithering in order to enhance a gray scale expression capability. In the case of carrying out an error diffusion, an artifact generated at an smooth area, for example, the background area due to an error component diffused into adjacent cells can be observed by a human eye. Since the multitoning technique has been basically developed for a printer, an application of the multitoning technique to the PDP violates its standard process. More specifically, since a printer has a small pixel size, error components between neighborhood pixels are averaged and hence an artifact is almost not observed by a human eye. On the other hand, since a PDP has a relatively large pixel or cell size, an averaging of error components is not made, but an error diffusion value of each cell is recognized by a human eye in the case of applying the multitoning technique to the PDP, and hence an artifact is prominently observed. Moreover, if total sustain pulse number only is adjusted in accordance with an average brightness of an input image like the conventional sustain pulse number control scheme to thereby increase the number of sustain pulses corresponding to one gray level interval to more than three, then an application of the multitoning technique causes more conspicuous observation of an error diffusion artifact.
In order to overcome such problems, there has been suggested a sustain pulse number control strategy of adjusting total sustain pulse number and, at the same time, determining total gray level number depending upon a difference between the highest brightness value and the average brightness value of an image after searching the brightest value from an input image. This sustain pulse number control strategy increases total gray level number when an average brightness of an image is low and a difference between the average brightness value and the highest brightness value is small, to thereby reduce the number of sustain pulses corresponding to one gray level interval, because an error diffusion artifact is well observed if total gray level number is small and total sustain pulse number is large when an average brightness of an image is low and a difference between the average brightness value and the highest brightness value of an image is large Otherwise, it decreases total gray level number when a difference between the average brightness value and the highest brightness value of an image is large. However, such a sustain pulse number control strategy still emerges an error diffusion artifact at a dark screen. Further, the conventional sustain pulse number control strategy differentiates a sub-field arrangement and an emitting pattern according to the number of gray levels when total gray level is varied in accordance with an average brightness of an input image. This causes an increase in a data capacity stored in a memory when the number of sub-field arrangements and emitting patterns as great as types of total gray level number is required. In addition, the conventional sustain pulse number control strategy has a problem in that a probability of generating a flicker in accordance with the sub-field arrangements and the emitting patterns is large.